1. Field of the Invention
The present invention relates to a hub structure of a rotary-wing aircraft, and more particularly to a bearingless hub structure of a rotary-wing aircraft.
2. Prior Art
A bearingless hub structure of a rotary-wing aircraft generally includes a hub body having a plurality of radially extending flexible beams provided with flexible plate sections having a low rigidity in the flapping direction. Each of the flexible plate sections is connected or made continuous with a flexible portion which is flexible in the lead-lag direction. The flexible beam also has a twist-flex portion which is of a low torsional rigidity. As disclosed by the Japanese laid-open patent application No. 61-21894 which has been laid-open to public inspection on Jan. 30, 1986, the flexible plate section, the flexible portion and the twist-flex portion are formed in radially offset positions with each other so that Coupling between the lead-lag movements and the pitch changing movements can be eliminated. With this arrangement, it is possible to obtain a design flexibility.
The U.S. Pat. No. 4,676,720 discloses a structure which is an improvement of the structure disclosed by the laid-open patent application No. 61-21894 to obtain an improved damping characteristics. More specifically, as described in the aforementioned U.S. patent, the structure disclosed by the application 61-21894 has a tendency that the flexible beam is bent in an S-shaped configuration so that it is impossible to provide a satisfactory damping property in the lead-lag direction. As the result, it becomes difficult to prevent a instability on the ground and in the air.
In view of the problems, the U.S. patent proposes to provide the flexible portion which has a low rigidity in the lead-lag direction with two beam portions which are spaced apart at the radially inner ends in the lead-lag direction and decreased in the spacing toward radially outward direction. The beam portions are located radially inwardly with respect to the twist-flex portion and has radially outward ends which are continuously formed with the radially inner end of the twist-flex portion. The rotor blades are connected at the radially inner ends by means of a plurality of bolts to the radially outer end of the pitch housing.
In rotary-wing aircrafts, it is necessary to fold the rotor blades when they are brought into hangers. For the purpose, in the aforementioned structure, the bolts connecting each of the rotor blades to the pitch housing are removed except one bolt about which the rotor blade is turned so that the rotor blade is placed along the longitudinal axis of the aircraft in a position wherein the rotor blade does not extend laterally beyond the contour of the aircraft.
It should however be noted that in this structure the rigidity at the connection between the rotor blade and the pitch housing will be decreased in the lead-lag direction to an extent that a distortion may be produced at this connection under a lead-lag movement. Such distortion may cause a decrease in the effective stroke of the lead-lag damper so that there may be lack of damping capability. A further problem encountered in this structure is that the connection between the rotor blades and the pitch housing requires connecting bolts of a substantial size and huge fittings so that the structure becomes bulky and causes an increased aeronautical drag and weight. It should further be noted that when the rotor blades are being folded as described above, there may be a possibility that the rotor blades may be accidentally dropped in handling. If such accident occurs, the twist-flex portion of the flexible beam section may be excessively twisted to an extent that the portion may be broken.
In order to eliminate the problem, the flexible beam may be separated from the hub body at a portion radially inward of the flexible plate portion which has a low rigidity in the flapping direction and connected with the hub body through connecting bolts. This structure however has disadvantages. First, it should be noted that the portion radially inward the flexible plate portion. is subjected to a large bending load in the flapping direction due to the flapping movements. The connection at this portion therefore requires huge fittings which can withstand the bending load. Further, the connection provided in this portion will cause an interference between the rotor blades when the blades are to be folded.